In television production, it is often desirable to synchronize two or more television signals from separate sources precisely with one another to produce a composite image or for other purposes. For example, the image of a television reporter may be superimposed over an image associated with the story which he is discussing. Techniques for superimposing one image over part of another image, commonly referred to as "keying" are well known in the television art. These techniques however require precise synchronization between the two signals to be combined.
The need for synchronization arises from the nature of television signals. In an ordinary television monitor, the image is created by sweeping an electron beam across the surface of a phosphor screen in a predetermined pattern or "raster", usually a pattern of parallel horizontal lines. The information defining the picture is provided in "rasterwise" order. That is, the brightness values for various points on the screen follow one another in the video signal in the same sequence as the beam crosses those points in tracing the predetermined pattern on the screen. As the beam reaches each point on the screen, its intensity, and hence the brightness of the light emitted by the screen at that, point are controlled in accordance with the brightness value in the signal. In many common television systems, the raster is "interlaced" so that the beam first traces a series of "even" horizontal lines spaced one line apart from one another over the full vertical extent of the screen, then returns to the top of the screen and traces another series of "odd" horizontal lines in the spaces between the even lines. The even lines are commonly referred to as one "field" of the picture, whereas the odd lines constitute another field. The even and odd fields together constitute one "frame", i.e., a complete picture.
In many situations, such as in television studio productions, it is essential to combine several video signals from different sources. The video information representing the same point in the raster must be presented to the combining device at the same time. Each field of the two video signals to be combined must start at the same time, and each line in each signal must start at the same time as a line in the other signal.
To a certain degree, this synchronization can be achieved by synchronizing the various video signal sources, such as the various video cameras in a studio, to a synchronization signal from a master source in the studio, commonly referred to as a "genlock" signal. However, the signals passing from the various sources to a combining device are subject to delays in propagation through cables and intermediate signal processing devices. Differences in the delays encountered by various signals within a studio can affect the synchronization. Although these differences in delays may be as small as a few microseconds, they have an appreciable affect on the image. In a typical high definition television system, an entire line is traced in about thirty microseconds, so that a signal which is out of synchronization by ten microseconds would be shifted by about one-third of a line. If two signals out of synchronization by this amount were combined with one another, the resulting picture defect would be clearly visible to the viewer. Analog delay lines may be provided at the signal inputs of the combining device to alleviate these effects. These can be adjusted to match the delays encountered by the various signals and thus achieve precise alignment between the starting times of the lines in the various signals, commonly referred to as "H phase synchronization". These devices are troublesome and normally do not provide satisfactory results with high definition television signals.
Television signals may be handled and stored in digital form. The brightness information constituting each line is converted into a series of values, referred to herein as "pixel values" each representing the brightness of one pixel. In monochrome systems, each pixel value may consist of one byte specifying brightness. In certain color television systems, each pixel value may include several separate bytes of digital information, each representing the brightness of one primary color. In other color television systems, one or more bytes of a pixel value may represent the overall luminance of the pixel, whereas one or more additional bytes may represent the chrominance value. These pixel values can be stored in a digital memory, read out from the digital memory in rasterwise order and reconverted to an analog signal for display on a video monitor. Computer special effects systems can create images by computing the series of pixel values which, when read out and displayed, will result in a picture representing an artificial image. Similar systems can modify the pixel values representing a real image captured by a video camera and thereby modify the image. Because the video signal for a complete image includes a large amount of information, a memory for storing a complete frame of video information must have a substantial capacity, typically several megabytes. Such a memory must be capable of storing and retrieving the information at very high rates. Full frame video memories therefore are relatively costly devices.
Considerable efforts have been made in the art heretofore toward development of video signal synchronization devices using digital elements.
Long et al., U.S. Pat. No. 4,018,990 adjusts the timing of a video signal by converting the video signal from analog to digital format and then clocking the digitized video information through a shift register and into a small random access memory using clock pulses synchronized to the incoming video signal. The signal is read out from the random access memory using clock pulses synchronized to an external source. The delay encountered by the incoming video signal in passing through this system can be adjusted by selecting a relatively short path through the shift register or a relatively long path. Thus, the delay encountered by a signal passing through the system can be adjusted to compensate for transmission line delays and the like. Shirota et al, U.S. Pat. No. 4,677,499 likewise uses a shift register as a digital delay line, and takes video signals from various points along the shift register to vary the path length and hence the delay time encountered by the video signal in passing through the register.
Cooper, U.S. Pat. No. 4,532,541 utilizes charge coupled device (CCD) analog shift registers as delay lines. The incoming signal is written to three such devices and the signal is clocked out from each such device in accordance with an output clock having the desired synchronization. The three CCD devices are shifted alternatively from write mode to read mode so that at any given time one device is always in write mode and accepting incoming signals whereas another device is always in read mode, and is discharging the previously written signals. Arnstein, U.S. Pat. No. 4,118,738 converts the incoming signal to a pulse train form, transmits the resulting pulse train through a digital delay line having multiple taps, and varies the delay encountered by the signal by selecting the appropriate tap from the delay line.
Tallent et al, U.S. Pat. No. 3,900,885 uses a set of three line memories in a frequency correction device. The incoming signals are digitized and written into the line memories in sequence at a writing rate synchronized with the incoming video signal. The signals are read out of these line memories either at a rate derived from a master reference signal or at a rate derived from the incoming video signal itself. In an alternate mode of operation, the digitized signals are routed around the line memories and the line memories are disabled.
Hopkins, Jr., U.S. Pat. No. 4,134,131 synchronizes video signals by reading them into and out of a full field memory or full frame memory. The incoming signals are digitized and the resulting pixel values are written into the memory and read out from the memory according to read and write signals synchronized with the master timing or genlock signal. The incoming signals are retained briefly in a buffer until a write signal occurs. Hashimoto et al, U.S. Pat. No. 4,916,541 discloses a video picture processing system incorporating a field memory together with input and output buffers.
Akiyama, U.S. Pat. No. 4,841,379 describes yet another system using a large memory with a write clock synchronized to the incoming signal and a read clock synchronized to a master signal.
Wells, II et al, U.S. Pat. No. 4,646,151 describes a system in which an incoming video signal is digitized and the resulting pixel values are written to first in first out or "FIFO" registers. The pixel values are written from the FIFO registers into a frame buffer memory and read out from the frame buffer memory to provide the output. The reading and writing operations are synchronized with the master synchronization signal. If the timing of the incoming video signal differs from the master synchronization signal, the difference will be corrected as the signals pass through the FIFO registers and encounter varying delays therein.
Sawagata, U.S. Pat. No. 4,095,259 discloses a video synchronization system using line buffer memories, with digitized video being written into the line buffers in synchronism with the incoming video signal and read out from the line buffers in synchronism with the master signal.
Tatomi, U.S. Pat. No. 4,063,284 discloses another system using shift registers with a write clock synchronized to the incoming signal and a read clock having a standard or master frequency. Sonoda, et al, U.S. Pat. No. 4,862,269 discloses another system using a memory with read in at a rate synchronized to one video signal and read out synchronized to another, master video signal.
Shinada, U.S. Pat. No. 4,802,025 discloses a video synchronizer using a pair of field memories and an interleaved reading and writing scheme. At any given time, the digitized incoming signal is written into one of the field memories whereas the outgoing signal is generated by reading data from the other field memory. Data writing is conducted according to clock signals synchronized with the incoming signal, whereas data is read out from the memory according to read clock signals synchronized with the master synchronization signal. This general scheme has been used in the high definition television art. Although this system provides effective synchronization, it is costly and is not particularly versatile. Thus, the memories included in such a system simply act as a synchronizers and perform no other function.
Despite all of these efforts towards improvements in video digital processing and synchronization, there are still needs for further improvement. In particular, there have been needs for video signal processing systems which can be used for several different functions including signal synchronization, mixing and frame storage, but which provide these functions at relatively low cost.